Scottish actor Gerard Butler celebrated his 41st birthday in style by performing an outrageous version of “Mustang Sally” with John Mayer.

The heartthrob movie star took over new restaurant The Darby on Saturday night with a dinner and put on a series of shows for 100 guests, including Adrien Brody, Kate Hudson, Bono and Jay-Z.

Onstage, Butler joined the 14th Street dinner club’s ‘mistress of ceremonies,’ cabaret star Lady Rizo a k a Amelia Zirin-Brown, who said: “Some of you know him as Gerry, other as Gerard. I know him as Mistress Butler.”

A spy told us: “Australian comedian James Smith toasted Gerry. He then went on to make fun of him saying: ‘Gerard has been out so much he’s now just constantly on Page Six. Gerry is on Page Six so much they have to make it Page Seven.’ Smith also joked that Butler and Jennifer Aniston had “as much sexual chemistry as Regis and Kelly” in “The Bounty Hunter.”

“Gerry also sang Oasis hit ‘Wonderwall’ with Lady Rizo and Mayer played guitar. They then followed it up with ‘Mustang Sally.’ He had a pretty good voice — it wasn’t what you’d expect from the guy from ‘300.’ It was epic. He was also in the deejay booth a lot and got the deejay to play a lot of rock, ’80s and funk.

“Gerry announced to the crowd that Mayer was going to sing for them, but John replied he just wanted to play guitar while Lady Rizo sang. So he had the crowd cheer to hear Mayer perform. Mayer then sang his hit “Who Says.”

Other guests at the dinner club, which officially opens on Wednesday, included skateboarder Tony Hawk, Jason Statham, Melissa George, Russell Simmons, Sante D’Orazio, Jeffrey Ross, Paul Haggis and a gaggle of models. A rep for Butler didn’t get back to us.